Measurements of equilibrium charge states of Br, I, Ta and U up to about 200 MeV

Volume 24B, n u m b e r 6

PHYSICS

LETTERS

20 M a r c h 1967

W e w o u l d l i k e to t h a n k M r . A. G o b b i f o r p r e paring the thin carbon foils.

this transition. One expects therefore a lower t r a n s i t i o n p r o b a b i l i t y f o r t h e 2 to 6 t r a n s i t i o n . The situation will be improved in the near future by using a magnetic shield around the transition magnet. T h e m e a s u r e d v a l u e of P 3 3 = - 0 . 9 3 5 ± 0 . 0 2 9 f o r t h e 3 to 5 t r a n s i t i o n i s w i t h i n t h e s t a t i s t i c a l u n c e r t a i n t y e q u a l to t h e c a l c u l a t e d m a x i m u m of - 0 . 9 6 2 f o r t h e p r e s e n t f i e l d in t h e i o n i z e r . In v i e w of t h i s r e s u l t o n e c a n s e t a n a r r o w u p p e r l i m i t o n t h e a m o u n t of d e p o l a r i z a t i o n in s u c h a c a r b o n foil. T a k i n g i n t o a c c o u n t a r e d u c t i o n of t h e p o l a r i z a t i o n of 2% b y the i n c o m p l e t e s e p a r a t i o n of t h e e l e c tronic states in the atomic beam one obtains a depolarization by the charge exchange process

Refe~'ences

In c o n c l u s i o n , t h e e x p e r i m e n t s h o w s t h a t o n e c a n o b t a i n t r a n s i t i o n p r o b a b i l i t i e s a s h i g h a s 100% a n d t h a t t h e d e p o l a r i z a t i o n by t h e c h a r g e e x c h a n g e in a foil is very small.

1. W. Haeberli, W. Griiebler, P. E x t e r m a n n and P. Schwandt, Phys. Rev. L e t t e r s 15 (1965) 267. 2. W. Griiebler, W. Haeberli and P. Sehwandt, Phys. Rev. L e t t e r s 12 (1964) 595. 3. H. Rudin, H.R. Striebel, E. B a u m g a r t n e r , L. Brown and P. Huber, Helv. Phys. Acta 34 (1961) 58. 4. R. Beurtey, Proc. 2nd Intern. Symp. on P o l a r i z a t i o n phenomena of nucleons, K a r l s r u h e , eds. H. Huber and H. Sehopper (Birkh~iuser V e r l a g Basel, 1966) p. 33. 5. A. A b r a g a m and J. M. Winter, Phys. Rev. L e t t e r s 1 (1958) 375. 6. A. Galonsky, H. B. Willard and T. A. Welton, Phys. Rev. L e t t e r s 2 (1959) 349. 7. J.M. Dickson, P r o g r e s s in Nuclear Techniques and Instrumentation 1, ed. F . J . H . F a r l e y (North-Holland Publ. Comp., A m s t e r d a m , 1965)p. 105.

MEASUREMENTS OF Br, I,

STATES MeV *

of (1 ~ 3)%.

Ta

OF EQUILIBRIUM CHARGE AND U UP TO ABOUT 200

L. G R O D Z I N S ,

R . K A L I S H a n d D. M U R N I C K

Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts and R. J . VAN D E G R A A F F ,

F. C H M A R A a n d P. H. R O S E

High Voltage Engineering Corporation, Robert J. Van de Graaff Laboratory, Burlington, Massachusetts Received 21 F e b r u a r y 1967

The c h a r g e distribution of Br, I, Ta and U ions passing through carbon foils and c e r t a i n g a s e s has been d e t e r m i n e d . The m o s t probable charge v e r s u s projectile energy is p r e s e n t e d for E p r o j up to ~ 200 MeV. The data can be used d i r e c t l y to predict the energy s p e c t r u m v e r s u s t e r m i n a l potential for an e l e c t r o static a c c e l e r a t o r .

In t h i s l e t t e r we p r e s e n t a p a r t i a l s u m m a r y of e x p e r i m e n t s to d e t e r m i n e t h e d i s t r i b u t i o n of e l e c t r o n s s t r i p p e d f r o m p r o j e c t i l e s of B r , I, T a a n d U passing through carbon foils and certain gases. * D r . R . V a n de Graaff died on 16 J a n u a r y 1967 and the world lost a b r i l l i a n t s c i e n t i s t and inventor. T h r o u g h out his c a r e e r he i n s p i r e d many i m p o r t a n t developm e n t s of the e l e c t r o s t a t i c a c c e l e r a t o r that b e a r s his name. This l e t t e r is a t e s t i m o n y to his most r e c e n t efforts to extend the scope of d . c . a c c e l e r a t o r t e c h niques. 232

The particles were accelerated in the High Voltage Engineering Corporation MP tandem accelera t o r . T h e i r e n e r g i e s , up to ~ 2 0 0 M e V , a r e t h e highest yet reported. This study has a two-fold interest. First, the d a t a s i g n i f i c a n t l y e x t e n d o u r k n o w l e d g e of s t r i p ping phenomena. Second, the data are important to t h e p r o d u c t i o n of b e a m s of e n e r g e t i c h e a v y i o n s . T h e n e e d f o r s u c h b e a m s i s w e l l k n o w n . It s t e m s f r o m t h e a d v a n t a g e s of i n c r e a s e d p r o j e c -

Volume 24B, n u m b e r 6

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Fig. 1. The charge s p e c t r u m obtained a f t e r p a s s a g e of a p r i m a r y iodine beam of charge state 24 and energy 154 MeV through a nominal 20 /.tg/cm 2 carbon foil (solid line), and through 1700 g c m of argon (dashed line). The slight d i s placement of peaks of the s a m e charge in the different t a r g e t s is due to the different energy l o s s e s in the foil and the gas. tile mass in such traditional studies as Coulomb e x c i t a t i o n a n d t h e e f f e c t i v e n e s s of h e a v y i o n s f o r t h e p r o d u c t i o n a n d i n v e s t i g a t i o n of n u c l e i in t h e r e g i o n Z > 100. F o r t h e t a n d e m a c c e l e r a t o r , t h e d a t a c a n b e u s e d d i r e c t l y to p r e d i c t t h e o u t p u t e n e r g y s p e c t r u m a s a f u n c t i o n of t e r m i n a l p o t e n t i a l . N e g a t i v e i o n s of a g i v e n e l e m e n t w e r e i n j e c t e d into the MP tandem accelerator. Input currents r a n g e d f r o m 1 - 5 0 h A f o r T a a n d U, a n d f r o m 1-5~tA f o r B r a n d I. T h e i o n s , a f t e r a c c e l e r a t i o n to t h e t e r m i n a l p o t e n t i a l , V T --< 10 M V , w e r e s t r i p p e d in a 5 ~ t g / c m 2 c a r b o n f o i l a n d a c c e l e r ated down the high energy tube. For higher output energies, further stripping could be obtained with carbon foils which could be inserted into the s e c o n d h a l f of t h e a c c e l e r a t i o n t u b e a t p o i n t s w h e r e t h e p o t e n t i a l s a m o u n t to T3 VT ' ~1 V T a n d ¼VT. A b e a m , h o m o g e n e o u s i n m a s s , e n e r g y and charge, was selected by a magnet followed by an e l e c t r o s t a t i c a n a l y z e r of 17.4 m r a d i u s a n d 20 ° arc. The resulting beam was passed through carb o n f o i l s r a n g i n g f r o m 5 to 20 g g / c m 2 o r a g a s cell containing either hydrogen, oxygen or argon, w h o s e t h i c k n e s s c o u l d b e v a r i e d f r o m 50 to 40 000 ~tcm. T h e r e s u l t a n t c h a r g e d i s t r i b u t i o n

was then analyzed by a second magnet. The field in t h i s m a g n e t w a s s w e p t s o t h a t t h e b e a m s of d i f ferent charge state passed across a surface barr i e r h a v y i o n d e t e c t o r p l a c e d a t 26 ° to t h e p r i m a r y b e a m d i r e c t i o n . A v o l t a g e p r o p o r t i o n a l to t h e m a g n e t i c f i e l d w a s g a t e d by t h e o u t p u t of t h e d e tector and addressed a multi-channel analyzer so that the charge distribution was automatically r e c o r d e d *. T y p i c a l s p e c t r a f o r 154 M e V i o d i n e i o n s of c h a r g e s t a t e 24 t r a v e r s i n g g a s ( d a s h e d l i n e ) a n d f o i l ( s o l i d l i n e ) a r e s h o w n in fig. 1. In t h i s e x a m p l e the t a r g e t s w e r e t h i c k e n o u g h to produce an equilibrium charge distribution, def i n e d h e r e a s t h e t h i c k n e s s n e c e s s a r y to s t a b i l i z e the most probable charge. In fig. 2 t h e m o s t p r o b a b l e i o n i z e d f r a c t i o n ~JZ, f o r B r , I, T a a n d U is p r e s e n t e d a s a f u n c t i o n of E½. T h e m o s t p r o b a b l e c h a r g e s t a t e h a s a r e l a t i v e i n t e n s i t y of a b o u r 20% in a l l c a s e s ; v a r i a t i o n s i n t h i s n u m b e r s e e m to b e c o r r e l a t e d with shell effects. The present data are shown as * The u r a n i u m data were taken in an e a r l i e r experiment with a different source injector s y s t e m and without the e l e c t r o s t a t i c analyzer. The method will be reported in a l a t e r paper. 283

Volume24B, number 6

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Fig. 2. The most probable ionized fraction, ~-/Z, for passage through an "equilibrium" thickness (see text) of carbon foil as a function of E½ of the incoming ions. Open symbols a r e data taken from r e f s . 2, 3 and 6. s o l i d s y m b o l s , t h a t of p r e v i o u s w o r k e r s b y o p e n symbols [1-3,6]. The data agree quite well in the o v e r l a p r e g i o n s . T h e e r r o r b a r s on o u r p o i n t s i n d i c a t e a n u n c e r t a i n t y of a b o u t ½ a c h a r g e u n i t f o r B r a n d I, a n d a b o u t o n e c h a r g e u n i t f o r T a a n d U. A d e t a i l e d d e s c r i p t i o n of t h e e x p e r i m e n t a n d t h e results for gas as well as foil stripping will be p r e s e n t e d e l s e w h e r e . S o m e of t h e c o n c l u s i o n s from this work are the following. (1) T h e U d a t a c o n f o r m w e l l to t h e e n e r g y d e p e n d e n c e of ~ / Z u s e d by B e t z e t al. [2]; E1/Z = = 1 - C exp(5/3/~). T h e f i t i s not a s g o o d f o r t h e I, B r a n d T a d a t a . F o r I, a t e n e r g i e s a b o v e 120 M e V , ~ i s l o w e r t h a n t h i s p r e d i c t i o n . (2) O v e r t h e e n e r g y r a n g e s t u d i e d , t h e f o i l t h i c k n e s s to r e a c h e q u i l i b r i u m i n c r e a s e s f r o m b e l o w 5 ~ g / c m 2 to o v e r 20 p g / c m 2. T h e e f f e c t is also observed in gases. For example, with I i o n s t h e e q u i l i b r i u m t h i c k n e s s of o x y g e n a n d a r g o n i n c r e a s e s f r o m < 1.5 ~ g / c m 2 a t 25 M e V to 10 ~ g / c m 2 a t 180 M e V ; t h e r e s p e c t i v e i n c r e a s e f o r h y d r o g e n i s f r o m ~ 0.15 ~ g / c m 2 to >> 5 ~ g / c m 2. (3) T h e s h a p e of t h e c h a r g e d i s t r i b u t i o n a t e q u i librium in foils and gases closely approximates a G a u s s i a n c u r v e , f (q) = A(q) e x p { - ( q - ~)2/2~2}. T h e w i d t h , ~, i n f o i l s in n e a r l y i n d e p e n d e n t of t h e e n e r g y of t h e p r o j e c t i l e ( f r o m 50 to 200 M e V ) a n d v a r i e s l i t t l e w i t h t h e Z of t h o s e p r o j e c t i l e s s t u * F u r t h e r i n c r e a s e of the gas thickness enhances the wings of the charge distribution with little effect on

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Fig. 3. The most probable output energy f r o m a tandem a c c e l e r a t o r with four carbon s t r i p p e r foils as a function of Z and for different t e r m i n a l potentials. The c r o s s hatched a r e a indicates the output energy envelope for V T = 16 MV. Also shown on the figure a r e the Coulomb b a r r i e r s for Z p r o j v e r s u s a U t a r g e t and Z p r o j v e r s u s a t a r g e t of s i m i l a r Z . d i e d . A t 1 0 0 M e V , ~ B r = 1.9 + 0.15, ~ I = 1 . 7 + 0 . 1 , ~ T a = 2 . 1 + 0.1, a n d ~U = 1.6 =L 0.2. T h e B r v a l u e f i t s t h e e m p i r i c a l p r e d i c t i o n of N i k o l a e v et al. [5] q u i t e w e l l . D e v i a t i o n s of the c h a r g e d i s t r i b u t i o n from a pure Gaussian shape and the slight depend e n c e of ~ w i t h e n e r g y , a p p e a r to b e c o r r e l a t e d with atomic shell structure; ~ is smallest when Z - ~ is a c l o s e d s h e l l . U s i n g t h e d a t a of fig. 2 we c a n c o n s t r u c t fig. 3, where the most probable output energy is given as a f u n c t i o n of t h e p r o j e c t i l e e l e m e n t , Z , f o r v a r i o u s t e r m i n a l p o t e n t i a l s r a n g i n g f r o m 8 to 20 MY. A four-stripper configuration has been assumed; t h e e n e r g y w o u l d b e l e s s t h a n 10% l o w e r if t h r e e s t r i p p e r s w e r e u s e d . If t h e p o t e n t i a l d i f f e r e n c e s between the foils are equal, then the energy dist r i b u t i o n c o n s i s t s of n e a r l y m o n o e n e r g e t i c c o m p o n e n t s s p a c e d ¼V T a p a r t . F o r V T = 16 M V , t h e a p p r o x i m a t e d i s t r i b u t i o n of o u t p u t e n e r g i e s i s i n d i c a t e d by t h e c r o s s - h a t c h e d a r e a b o u n d e d b y d a s h e d l i n e s r e p r e s e n t i n g a p a r t i c l e i n t e n s i t y of 0.01% of t h e b e a m i n c i d e n t on t h e t e r m i n a l foil. A l s o s h o w n in fig. 3 a r e c u r v e s f o r t h e C o u l o m b b a r r i e r s f o r t h e p r o j e c t i l e Z v e r s u s a t a r g e t Z, and for the projectile Z versus uranium. A nu-

Volume24B, number 6

PHYSICS

LETTERS

20 March 1967

1

c l e a r r a d i u s R = 1.5A~ f m i s a s s u m e d . W i t h a t e r m i n a l p o t e n t i a l of 16 M V , b r o m i n e c a n a t t a i n ~ 5 MeV/AMU and can overcome the Coulomb b a r r i e r of u r a n i u m .

1. A s u m m a r y of stripping data up to J a n u a r y 1966 is contained in P. H. Rose and A. Galejs, P r o g r e s s in Nuclear P h y s i c s and Instrumentation, ed. F . J . M . F a r l e y (North-Holland Publ. Co.. A m s t e r d a m ) , Vol.2 (1967). 2. H . D . B e t z , G.Hortig, E . L e i s c h n e r , Ch. Sehmelzer. B . S t r a d l e r and J. Weihraugh, Phys. L e t t e r s 22 (1966) 643. 3. E . A l m q u i s t , C.Broude, M . A . C l a r k , J . A . K u e h n e r and A . E . L i t h e r l a n d , Can. J . P h y s . 4 0 {1962) 954. 4. N . O . L a s s e n , Phys. Rev. 69 (1946) 137. 5. V.S. Nikolaev, I.S. Dmitriev, L. N. Fateeva and T a . A . T e p l o v a , Zh. Eksp. i T e o r o F i z . 3 9 (1960) 905, Soviet Phys. J E T P .12 (1961) 627. 6. A . E . L i t h e r l a n d , E . A l m q u i s t , B . H . A n d r e w s , C. Broude and J . A . Kuehner, Bull.Am. Phys. Soc. 8 (1963) 75.

T h e a u t h o r s w i s h to a c k n o w l e d g e t h e v a l u a b l e a s s i s t a n c e of C. H o r t i g , who w a s w i t h u s d u r i n g a n e a r l y s t a g e of t h e e x p e r i m e n t , a n d of F. F l a s a r a n d J. U d i n s k y f o r a s s i s t a n c e in t a k i n g a n d a n a l y z ing t h e d a t a . We a r e a l s o i n d e b t e d to t h e H i g h V o l t a g e E n g i n e e r i n g s t a f f u n d e r t h e d i r e c t i o n of N. B r o o k s a n d R. F e r n a l d f o r t h e a s s i s t a n c e in o p e r a t i n g the i o n s o u r c e a n d t a n d e m , r e s p e c t i vely.

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Nous avons m e s u r 6 p a r une mdthode bas6e sur l'effet Doppler, la vie moyenne du niveau de 3.085 MeV du noyau 13C. C e l l e - c i est de 55 ± 6 fs.

E n s u p p o s a n t l a s y m ~ t r i e d e c h a r g e et e n n ~ g l i g e a n t l e s e f f e t s c o u l o m h i e n s , M o r p u r g o [1] a m o n t r ~ q u e l e s p r o b a b i l i t ~ s d e t r a n s i t i o n de m u l t i p o l a r i t @ E1 e n t r e l e s ~ t a t s a n a l o g u e s d e s n o y a u x miroirs doivent ~tre rigoureusement ~gales. Cep e n d a n t W a r b u r t o n e t al. [2] o n t m o n t r ~ q u e c e t t e r ~ g l e ~ t a i t v i o l ~ e d ' u n f a c t e u r 2.4 × 1 0 - 3 d a n s le c a s d e s t r a n s i t i o n s E1 p r o v e n a n t d e s ~ t a t s ±+ de 8.31 M e V et 7.55 M e V e t c o n d u i s a n t a u x ~ t a t s f o n d a m e n t a u x ~ d e s n o y a u x 15N e t 150 . I1 6 f a i r d o n c i n t 6 r e s s a n t de m e s u r e r l a v i e m o y e n n e du n i v e a u de 3 . 0 8 5 M e V (½+) du n o y a u 13C p o u r l a c o m p a r e r ~t c e l l e du n i v e a u c o r r e s p o n d a n t d e 2 . 3 6 5 M e V du n o y a u 13N [3]. Pour ceci nous avons utilis~ une m6thode bas6e sur l'effet Doppler, consistant ~ calculer la forme de l a r a i e du r a y o n n e m e n t g a m m a 6 m i s a u c o u r s du r a l e n t i s s e m e n t du n o y a u . C e l l e - c i d ~ p e n d de l a v i e m o y e n n e de l ' ~ t a t i n i t i a l d e l a t r a n s i t i o n , du p o u v o i r d ' a r r ~ t du r a l e n t i s s e u r p o u r le n o y a u 6 m e t t e u r c o n s i d e r S , et d e s c o n d i t i o n s i m p o s @ e s par la cin6matique de la r6action nucl6aire cond u i s a n t ~ c e n o y a u . De p l u s , l a f a i b l e e f f i c a c i t 6 d e n o t r e d ~ t e c t e u r n o u s o b l i g e fi t e n i r c o m p t e de i

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l a d i s t r i b u t i o n a n g u l a i r e d e s n o y a u x de r e c u l . L a c o m p a r a i s o n du s p e c t r a a i n s i c a l c u l ~ a u s p e c t r e experimental mesur~ grace ~ une diode Ge(Li) c o n d u i t ~ l a v a l e u r de l a v i e m o y e n n e . L e n i v e a u d e 3.085 M e Y du n o y a u 13C ~ t a i t aliment~ par la r~action 12C(d,p)13C ~ une ~nerg i e d e s d e u t o n s i n c i d e n t s de 2 . 8 0 0 MeV. A c e t t e ~ n e r g i e de b o m b a r d e m e n t , l e s n o y a u x 13C, d a n s l'~tat considerS, sont ~mis dans un cbne vers l ' a v a n t . C e c i c o n d u i t ~t u t i l i s e r u n e c i b l e m i n c e d e 12C (3 k e V p o u r d e s p r o t o n s de 1.7 M e V ) ~ v a p o r ~ s u r u n s u p p o r t de t a n t a l e . D a n s u n e b o n n e a p p r o x i m a t i o n , s e u l le s u p p o r t c o n t r i b u e a u r a l e n t i s s e m e n t d e s n o y a u x de r e c u l . P o u r e x p l o i t e r n o s r ~ s u l t a t s , n o u s n o u s s o m m e s s e r v i s de la distribution angulaire mesur~e par McEllistrem e t al. [4]. L e s r a y o n n e m e n t s g a m m a ~ t a i e n t o b s e r v e s ~ 0 ° p a r r a p p o r t / t l a d i r e c t i o n du f a i s c e a u incident ~ l'aide d'un d~tecteur Ge(Li) de 5 cm 3 de v o l u m e s e n s i b l e , p l a c ~ ~ 5 c m de l a c i b l e , qui d o n n a i t u n e l a r g e u r ~i m i - h a u t e u r de 7 k e Y p o u r l a r a i e d e 661 k e y du 1 3 7 C s . C e t t e p o s i t i o n e s t utilis@e p o u r ~ v i t e r d e s c a l c u l s c o m p l i q u ~ s e n g a r d a n t l a s y m ~ t r i e c y l i n d r i q u e de l a r ~ a c t i o n

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